A load cell is a device used to provide measurement of an applied force. In general, a load cell is subjected to the applied force and then converts the force into a measurement. This conversion can be achieved, for example, mechanically, hydraulically, or electronically. The type of load cell suitable for a particular application is dependent upon, among other things, the magnitude of the force to be measured, the rate of application of the force, and the environment in which the load cell will be used.
An electronic load cell, for example, converts a force into an electrical signal proportional to the force. This is usually done with a mechanical arrangement that deforms in response to the applied force. The mechanical arrangement is fitted with one or more strain gauges to convert the applied force into an electrical signal. This can be done, for example, with a piezoresistive element, the resistance of which changes in response to an applied force. The applied force can be measured by passing a reference voltage through the piezoresistor and measuring the change in voltage due to the applied force. The applied force can be calculated from the change in the reference voltage using an appropriate algorithm.
In other applications, it may be desirable to use a hydraulic load cell. A hydraulic load cell uses a chamber filled with hydraulic fluid attached to a hydraulic pressure gauge. When the load cell is subjected to an applied force, the chamber exerts a force on the hydraulic fluid producing a reading on the hydraulic pressure gauge. This hydraulic pressure can be converted into a force based on the area of the chamber. Hydraulic load cells can be useful for outdoor applications because they are not affected by spurious voltages, such as lightning. This can be useful in applications such as over-the-road truck scales.
Conventionally, load cells have generally been used to measure only axial forces, i.e., forces that act parallel to the central axis of the load cell. The aforementioned truck scales, for example, are used to measure the weight of a truck (or a downward force) as it passes over vertically oriented scales. Load cells are commonly used to measure the thrust provided by a jet or rocket engine. Engine test stands are intentionally designed hold the engine substantially in line with the load (and to measure only axial forces) to preclude the need to contain any side forces produced by engine misalignment.
In some applications, however, it can be desirable to measure both normal and shear forces exerted on a load cell. In the development of tires for both racing and consumer applications, for example, it is desirable to measure the traction provided by the tire. The traction provided by a tire is the amount of shear force the tire can generate against a road or other surface before it breaks free, and spins or slides. Tires generate longitudinal shear forces, for example, when a vehicle is accelerating or decelerating in a straight line. Tires generate lateral shear forces, on the other hand, when the vehicle is turning.
Tires are often tested, for example, on a skid pad, which is a circle of known radius painted on a testing surface such as asphalt. A test driver drives a car fitted with a particular tire around the circle while increasing his speed. Up to the point the car breaks loose, the centripetal force being generated by the speeding car is being offset by an equal and opposite shear force generated by the tires against the asphalt. The maximum such shear force occurs just before the tires break loose. The speed of the test vehicle at this point can be used to calculate the road holding ability of the tires (on that car), which is usually calculated in g-forces. Similar tests can be performed by braking and accelerating the car at the limits of traction. All of this information can be used to develop new rubber compounds that, for example, provide increased traction, increased tread life, or a combination of the two.
This type of testing, however, does not accurately account for the fact that the traction of a tire increases as the normal force with which the tire is forced into contact with the surface increases. In other words, a tire mounted on a heavier car will be less likely to spin at the same level of power. It can be desirable, therefore, to plot the shear force being generated by the tire against the normal (or axial) load that is applied to the tire or to remove the axial load from the data altogether. This provides researchers with a substantially objective measure of traction for a particular tire compound with the axial load removed.
This can be useful to compare the outright traction afforded by different compounds and/or tire constructions. This normalized data can also reveal tire compounds and/or constructions that may, for example, be suited for lighter cars, but not heavier ones and vice-versa. Tires can also be tested in a variety of applications designed to simulate real world driving to arrive at a tread compound and/or tread design that performs as desired for a particular job or set of jobs.
As shown in FIG. 1, one example of such an application is measuring the traction of a tire 105 against a surface 110 in a single direction. The surface 110 could be, for example and not limitation, the drum of a dynamometer 110, the surface of a rolling road, or a track surface. The maximum traction of the tire 105 against the roller 110 is measured as the maximum shear force FS the tire can generate. The shear force FS, while affected by other factors such as temperature, is primarily dependent upon two basic parameters: (1) the coefficient of friction between the tire and the drum and (2) the axial force FA applied to the tire. The maximum frictional force FS that can theoretically1 be applied by the tire 105 to the drum 110 is given by the formula:FS=μSFN Where μS is the static coefficient of friction between the tire 105 and the drum 110 and Fn is the normal force, which in this case is equal to the axial force FA. Any rotational force applied to the tire (e.g., by an engine through a transaxle) that is below this threshold can be applied by the tire 105 to the drum 110 without slippage. Any rotational force applied to the tire 105 that exceeds this frictional force FS will begin to spin the tire 105 on the drum 110 resulting in a loss of traction. This loss of traction is due to the fact that the frictional force of the tire once it begins to spin is dictated by the kinetic coefficient of friction μK, which is smaller than μS for most materials. 1 The actual force that can be applied by the tire, while linear, is slightly lower than the theoretical amount due to tire heating, deformation, and other factors.
Increasing or decreasing the coefficient of friction between surfaces is an important component in product design. For example, in automotive applications, increasing the traction between the tires of a car and the road surface results in, among other things, improved handling, acceleration, and safety. On the other hand, decreasing friction between bearing surfaces, for example, can improve fuel economy and decrease wear. As discussed above, however, because frictional force FS is based in part on the force normal to the surface, or FA in this case, accurate measurement of frictional forces requires measurement of both axial forces and shear forces. What is needed is a compact, inexpensive load cell capable of measuring multiple forces at the same time. It is to this end that embodiments of the present invention are primarily directed.